This invention relates generally to nonaqueous rechargeable electrochemical cells and particularly to rechargeable lithium-ion cells and to a novel negative electrode material having improved capacity and cycle life for use in such cells.
The negative electrode material in state of the art lithium-ion cells consists of carbon (graphite) as its active material, bonded to a metallic current collector with a polymeric binder. In a lithium-ion cell, this negative electrode material serves to reversibly accept lithium through an intercalation or alloying process. Intercalation typically occurs in the case of layered or tunneled materials whereas alloying may occur in metallic materials.
To improve the performance of lithium-ion cells, including cell capacity and cell safety, materials with a capacity higher than graphite and with improved safety properties are required. When lithium is intercalated into graphite upon charging of a lithium-ion cell, its potential approaches that of lithium metal, raising concerns about the safety of the lithium-ion cell. Further, the reversible capacity possible with graphite electrode materials is currently limited to about 90 percent of the theoretical capacity of graphite or about 335 mAh/g, (740 mAh/ml).
With a view towards achieving higher cell capacity as well as safer negative electrode materials, there has been renewed interest in metallic materials that can reversibly accept lithium, particularly those which utilize the reversible reaction of lithium with tin as their basis. Such materials were originally investigated as electrode materials for lithium/metal sulfide batteries while yet further efforts pursued alloys of lithium with aluminum, tin, antimony, lead, indium or bismuth towards monolithic electrodes for lithium batteries. These efforts were not satisfactory, however, since the electrodes failed mechanically, crumbling upon repeated cycling. Efforts by Yang et al., for example, as disclosed in xe2x80x9cSolid State Ionics 90xe2x80x9d (1996), pages 281-287, found that the mechanical failure of metallic negative electrode materials was highly dependent on the metallic matrix morphology and that the failure of the material can be partially mitigated when two-phase metallic matrices are utilized.
Other recent efforts towards a tin based negative electrode material have focused on the development of materials based on tin oxide, oxides containing tin, or mixtures of tin oxide with other materials. Such materials were the subject of extensive investigation, for example, by Idota et al as reported in U.S. Pat. No. 5,618,640 and European Patent No. 0 651 450 B1. Similarly, others have investigated the utility of tin oxide as an electrode material in lithium-ion cells (T. A. Courtney et al., Journal of the Electrochemical Society 144 (1997), pages 2045 to 2052).
As is known in the prior art, the reversible process in tin oxide materials involves the alloying of tin metal with lithium. On the first cycle, oxygen in the material has been shown to react irreversibly with lithium to form lithia. Although the lithia is electrochemically inert, materials with reduced lithia content, such as those prepared from SnO, have to-date not offered the reversibility of those with more lithia, such SnO2. The lithia has been proposed to accommodate the 300% volume expansion of the tin particle as it incorporates lithium, preventing mechanical failure of the material and promoting long cycle life. While tin oxide materials offer high specific capacity, e.g., 400 to 650 mAh/g, low capacity fade, less than 0.04 percent per cycle, good safety properties and broad materials compatibility, the high irreversible capacity on the first cycle, typically 1000 mAh/g for SnO2, has precluded the commercial success of tin oxide based materials. Nonetheless, these studies demonstrate the performance possible given a tin based negative electrode material.
To mitigate the irreversible capacity associated with tin oxides, efforts have recently focused on non-oxide tin materials. Some efforts have focused on the development of active/inactive composites wherein the active phase, tin or a tin compound, was incorporated into an inactive phase, such as a tin-iron material. These efforts sought to have the inactive phase serve the function of the lithia in the tin oxide materials to avoid the irreversible capacity associated with the formation of lithia. While these efforts demonstrate that a wide variety of tin containing materials can reversibly incorporate lithium, these materials either do not offer the capacity demonstrated with tin oxide or have associated with them high irreversible of the tin oxide materials.
According to the invention, an improved negative electrochemical cell comprising an intimate mixture of finely-divided elemental nickel and tin particles, characterized in that the oxygen content in said mixture is actually less than about 6 percent by weight of the mixture. Because of its low oxygen content, the negative electrode material of the invention has a small irreversible capacity and offers improved capacity and cycle life performance comparable to prior art tin oxide based materials. It is postulated that the nickel in this mixture, while inactive to lithium, does serve to accommodate the volume change of the tin as the tin incorporates lithium during charging of the cell.
In a more specific aspect, the invention provides an improved negative electrode for a rechargeable lithium-ion electrochemical cell comprising a mixture containing from about 5 to 90 percent by weight of finely-divided nickel particles and from about 10 to 95 percent by weight of finely-divided tin particles and having an oxygen content of less than about 2.3 percent by weight of the mixture. Based on the level of oxides that may be present in the material, mostly as surface contaminants or impurities, the particulate mixture may contain oxygen in amounts less than about 50 percent by weight and preferably less than about 1.0 percent by weight.
The negative electrode material of the invention is preferably prepared by mixing finely-divided nickel and tin powders in a ball or shaker mill inside a sealed container or vial under an inert gas such as argon. It has been found that when the powders are mixed in a ball to material weight ratio of about 1:1, the resultant material is essentially elemental nickel and tin but that when the ball to material weight ratio is substantially increased, say to about 8:1, the elemental nickel and tin powders alloy to form a nickel-tin compound, Ni3Sn2. This nickel-tin compound is both physically and chemically different and offers lower capacity and cycle life when compared to the elemental nickel-tin electrode material of the invention.
The nickel-tin electrode material of the invention is composed of discrete, non-spherical, smooth particles of a size ranging from about 1 to 10 micrometers (xcexcm). The particles have a density which is greater than about 5 grams per milliter and a specific surface area of less than about 1 square meter per gram. X-ray diffraction patterns taken on the material show the presence of peaks associated with elemental tin and nickel and the absence of peaks associated with the compound Ni3Sn2.
Electrodes for rechargeable lithium-ion batteries may be prepared by coating a suspension of the elemental nickel-tin mixture along with a polymeric binder onto a metal foil such as a nickel foil, for example.
The invention also contemplates the preparation of rechargeable lithium-ion electrochemical cells using negative electrodes composed of the elemental nickel-tin electrode material.